The present invention relates to surgical prostheses. The present invention is particular suited to a rotationally symmetrical surgical prosthesis such as an acetabular cup.
It is well known to repair bone joints damaged through disease or injury by implanting prosthetic components to replace part or all of the natural bone joint. For example, surgical reconstruction of a hip joint may require a femoral prosthetic component implanted at the end of the femur to replace the natural femoral head with a prosthetic bearing head and a prosthetic acetabular cup implanted within a reamed acetabular cavity or the natural acetabulum to receive the prosthetic bearing head.
There are a range of different fixation techniques known for securing prostheses to the surface of bones, or within bone cavities. Furthermore, these fixation techniques may be used in combination. Commonly, mechanical fixation is provided by securing the prosthesis with screws, pegs, wires or similar fasteners extending from the prosthesis into the bone. It is also known to provide a coating to a surface of a prosthesis which when implanted is in contact with the bone, or in close proximity to the bone, where the coating is chosen to promote osseointegration. Osseointegration is the direct structural and functional connection between living bone and the surface of a prosthesis. Osseointegration may either result from mechanical retention whereby bone ingrowth into surface features of a prosthesis, in particular a metal prosthesis, secures the prosthesis to the bone, or bioactive retention whereby the implant is coated with a bioactive material which stimulates bone formation leading to a chemical bond in which the implant is ankylosed with the bone.
Osseointegration by mechanical fixation occurs for a number of metals commonly used within implantable prostheses, such as titanium and titanium alloys. It can be encouraged by the provision of topological features like vents, slots and dimples upon the surface of the prosthesis in contact with the bone. There is no chemical retention of the prosthesis and the retention is dependent upon the surface area of the prosthesis.
Osseointegration by mechanical fixation may be encouraged by treating a surface portion of a prosthesis to increase its surface area, for example etching the surface. Alternatively, it is known to apply a porous coating to the metallic substrate such that bone ingrowth into the pores forms a firm bond between the prosthesis and the bone. The porous coating may consist of a plurality of small discrete particles of a metallic material bonded together at their points of contact to define a plurality of connected interstitial pores in the coating. Such a coating material, and a method of forming the coating, is described in U.S. Pat. No. 3,855,638. Preferably, the particles are of the same metallic material as the substrate. The coating may be formed by applying an adhesive to the portions of the substrate to be coated and applying the particles to the adhesive. Alternatively, a slurry of metallic powder suspended in an aqueous solution may be formed and applied to the substrate. The prosthesis is then sintered to remove the adhesive or aqueous solution and to fuse the particles together and to the substrate. Such a porous coating is commercially available from DePuy Orthopaedics, Inc under the name Porocoat.
Bioactive osseointegration occurs when the coating stimulates bone formation. A suitable coating material is hydroxyapatite (HA, also known as hydroxylapatite). Hydroxyapatite is a naturally occurring mineral form of calcium apatite which forms up to seventy percent of natural bone. Hydroxyapatite is commonly used as a filler to replace amputated bone or as a prosthesis coating to promote osseointegration.
Correct alignment of an implanted prosthesis relative to the natural bone, and in particular to any reamed cavity within the bone arranged to receive the prosthesis, is essential to ensure a strong bond to the bone and to achieve correct mobility of the reassembled joint. For instance, for an acetabular cup, a cavity is formed in the acetabulum (or if appropriate the natural acetabular cavity may be used) shaped to receive the cup (which generally has a hemispherical outer surface). The cup is intended to be positioned eccentrically within the cavity such that a portion of the cup protrudes above the rim of the cavity. However, inexperienced surgeons may mistakenly believe that the cup is intended to be seated and secured in position flush with the acetabulum rim in order to replicate the cups natural orientation. This is incorrect and can restrict the movement of the hip.
A second problem with conventional acetabular cup placement results from difficulty in positioning the cup owing to the surgeons viewing angle. To position an acetabular cup, typically the cup is inserted at an inclination of 40° relative to the patient's longitudinal axis. It is then typically necessary to apply 20° of anteversion (rotation about the patient's longitudinal axis) to assume a correct anatomic position. However, if the surgeon is viewing the patient on an anterior-posterior plane then when the operative anteversion is applied the inclination angle appears to increase. This compound angle effect could cause an inexperienced surgeon to compensate by reducing the inclination. However, this results in incorrect cup implantation, which increases the wear rate of the cup.
A third problem is that for conventional acetabular cups, when correctly placed in the cavity portions of the cup protruding from the cavity may comprise rough surfaces due to surface treatment of the substrate to promote osseointegration. The rough surfaces may abrade surrounding soft tissues.
It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere. In particular it is an object of embodiments of the present invention to provide a prosthesis which aids the surgeon in correctly positioning the implanted prosthesis by reference to local bone landmarks.